The ability to measure the Mueller matrix and/or the Stokes parameters of a target or a structure, and form images based on them, and then obtain their spectral image difference (dual-energy image subtraction), can provide significant insight on the sample/target composition. Such measurements can also reveal significant structural or molecular information that cannot be obtained via conventional imaging techniques. As a result, high-signal-to-background ratio, leading to an enhanced specificity, and high contrast images could be obtained by any imaging system that could apply the above principles. Furthermore, information could also be obtained by applying Mueller matrix decomposition, image-processing, neural-fuzzy logic algorithms and image reconstruction techniques that operate directly on the Mueller matrix and/or the Stokes polarization parameters.
The present invention is referred interchangeably through out the text as “Mueller Matrix/Stokes Parameters Polarimetric Spectral Difference Imaging” or “Mueller Matrix/Stokes Parameters Polarimetric Dual-Energy Imaging”, without loss of meaning, since it leads to the formation of several Mueller matrix polarimetric difference images and/or Stokes polarization parameter image differences, formed by pairs of Mueller matrix polarimetric images/Stokes polarization parameter images, acquired at at least two distinct wavelengths, chosen from a wavelength spectrum λ1, . . . λn. Therefore, multiple spectral polarimetric image differences can be obtained.
The present invention initially acquires images based on Mueller matrix and/or Stokes polarization parameter formalism/imaging principles through the interrogation of targets with multiple wavelengths. The present invention then subtracts these images, acquired at at least two different wavelengths to yield multi-wavelength (multi-spectral) polarimetric image differences. In other words, in one embodiment the present invention permits the fusing of multi-spectral difference detection principles with Mueller matrix and/or Stokes polarization parameter imaging principles. Further imaging information about the target/structure can be/is obtained by Mueller matrix polar decomposition of the images at different wavelengths and forming image differences at at least two wavelengths.
In fact, Mueller matrix measurements permit parameters such as diattenuation, retardance, depolarization power, and birefringence to be obtained. The importance of these parameters can be enhanced further under multi-spectral interrogation of the target/structure, providing useful information regarding the nature of the target/structure. For instance, interrogation of biological structures with multiple wavelengths, leads in practice to a multilayer interrogation of tissue, allowing one to obtain high-contrast images at different depths. This allows one to differentiate tumor and cancerous structures or cells from healthy ones based on a change in tissue birefringence. Therefore, a subtraction of the birefringence obtained at at least two distinct wavelengths can enhance the structure of interest, removing the influence of the interfering tissue or cells. Therefore subtraction of the diattenuation, retardance, depolarization power, and birefringence at distinct wavelengths, under multi-spectral interrogation of the target/structure can provide insightful structural and physiological information based on the difference of the attenuation of amplitude of the incident light, phase change difference, depolarizing potential of the target difference, and phase shift difference, due to the variation of the index of refraction, obtained at at least two distinct wavelengths, respectively.
The principles of the multi-fusion multi-spectral-dual-rotating retarder, dual-energy complete polarimeter, are shown in FIG. 1. However, the present invention can be applied to any theoretical or experimental technique that estimates the full-16 element Mueller matrix of the system (target/associated optics), and relates, therefore, the output Stokes parameters to the input Stokes parameters.
The principles multi-spectral Mueller matrix polarimetric image difference and/or Stokes polarization parameters image difference involve, in one instance, the acquisition of multi-wavelength optical Mueller matrix/Stokes polarimetric images. In one embodiment, a weighted subtraction of two Mueller matrix images, produced from a high energy (low wavelength) and another from a low energy (high wavelength) energy and/or light source can produce a polarimetric Mueller matrix/Stokes polarimetric image difference, which eliminates interfering background structure, as well as it enhances the polarization-based amplitude contrast information (diattenuation property of the target/structure), and polarization-based phase contrast information (birefringence property of the target/structure).
Polarimetric imaging offers distinct advantages for a wide range of detection and classification problems.
Polarimetric imaging relies on the preservation of polarization of backscattered light, while offering distinct signatures related to surface smoothness, orientation, and target/structure composition. Under certain circumstances, the polarization of the scattered light depends upon a number of geometrical, and physical parameters, such as incident polarization state, shape, size, and concentration of the scatterer, or more generally from the refractive indexes of the scatterer and the surrounding medium. Specifically, it relies on the assumption that weakly scattered light maintains its initial polarization state, while highly scattered light does not maintain its initial polarization state. In one embodiment, the present invention permits the use additional polarimetric-sensitive signatures, such as scattering, due to different concentration of the scatterer, size, and other variables to be obtained under multiple wavelength interrogation, and subsequently used to form a polarimetric image difference.